Abstract: To investigate the effects of substrate salinity on cabbage seedling growth and the rhizosphere microbial community, this study used cabbage cultivar “Xusheng” as the test material. Four NaCl concentration treatments including 0 (CK), 0.5 (L), 1.0 (M) and 2.0 g/L (H) were established in a seedling substrate with the same basic formula. Seedling agronomic traits were measured, and 16S rRNA high-throughput sequencing was employed to analyze structural and functional changes in the rhizosphere microbial community. The results showed that under the L treatment, leaf area, aboveground dry weight, and belowground dry weight of cabbage seedlings increased by 12.61%, 19.35% and 25.00%, respectively, compared with CK, and the robust seedling index increased by 18.30%, demonstrating the best seedling cultivation performance. The M treatment was broadly comparable to CK, whereas the H treatment significantly inhibited cabbage seedling growth and resulted in the lowest robust seedling index. The microbial community structure of the L treatment did not differ significantly from that of CK, and the L treatment potentially enhanced stress-resistant functions of the microbial community, such as nitrogen respiration, nitrate reduction, and aromatic compound degradation. In contrast, the H treatment markedly weakened both growth-promoting and stress-resistant functions, with the sulfate-reducing bacterium Thermodesulfobacteriota becoming the dominant taxon and posing a potential toxic risk to seedlings. In conclusion, in substrate-based seedling cultivation, low-concentration NaCl may modulate the stress-resistant functions of the rhizosphere microbial community, thereby stimulating the growth potential of cabbage seedlings to a certain extent, whereas high-concentration NaCl leads to rhizosphere micro-ecological imbalance and the proliferation of harmful bacteria, which is detrimental to cabbage seedling growth.